PD Testing and Monitoring of HV XLPE Cable Systems ENU

8/19/2019 PD Testing and Monitoring of HV XLPE Cable Systems ENU

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PD TESTING AND MONITORING OF HV XLPE CABLE SYSTEMS

W. KoltunowiczOMICRON Energy Solutions GmbH, Lorenzweg 5, Berlin 12099, Germany

M. KrügerOMICRON electronics GmbH, Oberes Ried 1, Klaus 6833, Austria

Abstract: The paper deals with on-site PD testing and continuous monitoring of HVXLPE cable systems and describes the method that can detect and locate partialdischarges at all accessories simultaneously. The example of after installation PDmeasurements procedure applied on 220 kV cable line is shown in the paper. Asconcerns continuous monitoring, an advance concept to assess and monitor thecondition of the insulation of a 420 kV cable system in a 10 km underground tunnel isdescribed. The advanced features for the elimination of disturbances and forseparation of different types of insulation defects based on synchronous, multi-channel and multi-frequency techniques are shown. The processing of monitoring data

is implemented in a modular software system that allows reliable long-term storage ofmonitoring data and provides remote access via a web interface. The separatechapters are dedicated to the procedure of site acceptance for the monitoring systemand also to system maintenance strategy.

1 INTRODUCTION

The correct design of the XLPE cable and itsaccessories, terminations and joints, is checked bythe type tests, and the quality of production ischecked by routine tests at the manufacturer’splant according to relevant standards.

Installation work on site poses an additional risk ofintroducing faults. Small particles, dust, andmoisture might lead to defects in electrically criticallocations of the accessories. Dielectric testsperformed on site do not replace type tests androutine tests. They are supplementary to dielectricroutine tests and are aimed at checking thedielectric integrity of the fully assembled cable linein order to eliminate defects, such as damagesduring transportation and lay-out or incorrectassembly of the accessories.

The preferred voltage for on-site tests is ac voltageof industrial frequency. Voltage testing delivers

only binary results (withstand or breakdown).Therefore, it is recommended to combine acvoltage testing with sensitive on-site partialdischarge (PD) measurements [1].

A major part of all in-service failures in HV XLPEcables can be attributed to the insulation system ofaccessories, joints and terminations. These failureswill normally develop over time. In order to detectthese changes at an early stage, detailedinformation about the actual insulation condition isnecessary. With suitable sensors, this informationcan be derived for example by monitoring PD

activity during the operating of the equipment.This paper describes the best practice forperforming after installation testing and continuous

PD monitoring to assess the quality of the HVXLPE cable system.

2 PD MEASUREMENTS DURING AFTER-INSTALLATION TESTING

High-voltage (HV) tests are executed on site for all

extruded HV cables. On-site test proceduresusually have to be negotiated between themanufacturer and the user on the basis ofinternational and national standards.

Two IEC standards cover after installation tests ofextruded cable systems: IEC 60840:2004 forcables of rated voltages from 30 kV (Um = 36 kV)up to 150 kV (Um =170 kV) and IEC 62067:2001 forrated voltages above 150 kV up to 500 kV(Um = 550 kV). High test power, especially requiredfor long cable lines testing, can only be effecientlygenerated by mobile resonant test systems, where

the weight-to-power ratio and feeding powerdemand is relatively low and the transport volumeis acceptable (Fig.1).

Figure 1 Resonance test set connected to the cabletermination



The whole dielectric test should be performed as astep test. PD measurements should be taken atevery voltage level. By increasing the test voltagein steps of e.g. 20% of the maximum test voltage,critical defects are usually identified beforebreakdown.

Because HV cables must be tested at themanufacturing facility prior to shipping to theinstallation site, the on-site PD measurements

focus on the field-installed accessories. For thispurpose, each accessory has to be equipped withspecial sensors - high frequency currenttransformers (HFCTs) to pick up the PD signal withhigh efficiency (Fig. 2). In the particular case ofafter installation testing of 220 kV XLPE cable line,fifteen HFCTs were used to pick up the PD signal.They were equipped with an air gap in the core toprevent magnetic saturation of the core.

Figure 2 PD test arrangement

One MPD 600 unit was connected to the couplingcapacitor and calibration was performed accordingto the IEC 60270 and IEC 60885 standards [4,5]. Aperformance check was performed on the otherfifteen PD acquisition units, which were mountedinto the link boxes close to the terminations andconnected in a daisy chain with fiber optics (Fig. 3).

A real calibration is not possible here becauseimpulses cannot be injected directly into the closedand buried joint.

Figure 3 Performance checks of HFCT sensors

The resonance test set used IGBT’s as switchingsemiconductors. They produce strong impulses

that cause high interference impulses on the MPD600 instruments, especially to those units whichare rather close to the resonance test set. Toeliminate this effect, PD signal gating wasperformed by one extra MPD 600 unit which wasinstalled close to the IGBT circuit to receive theswitching impulses. During the time of theswitching impulses, the signals of all other unitswere blocked by the MPD 600 software. Due to the

fact that this is done by software, such gating canbe deactivated also for the replay of all recordedstreams later on. The result of gating is shown inFig. 4.

Figure 4 PD measurements at joint without gating (left)and with gating (right)

The dielectric withstand test was performed at 180kV and the corresponding voltage frequency andcurrent were 26 Hz and 68 A respectively. Themeasurement center frequency for all MPD600units was set between 2 MHz and 3 MHz. Theselection of lower frequency was not possible dueto the high interference. The PD results at 180 kVat all MPD 600 units are shown in figure 5. No PDactivity was detected and the tests weresuccessfully finished within 3 ½ days.

Figure 5 PD measurement at 180kV with 15 MPD units – 1.1=coupling capacitor, 1.3=joint 1-2, 1.4=joint 2-3,1.5=joint 3-4, 1.6=joint 4-5, 1.7= joint 5-6, 1.8= joint 6-7,1.9=joint 7-8, 1.10=joint 8-9, 1.11=joint 9-10, 1.12=joint10-11, 1.13=joint 11-12, 1.14=joint 12-13, 1.15=endterminal of GIS

3 MONITORING OF 420 KV XLPE CABLESYSTEM in UNDERGROUND TUNNEL

A. Concept of the Monitoring System

An underground tunnel 10 km in length and having

a 3 m inner diameter connects the substations atBeddington and Rowdown in London. The tunnelhouses a new 400 kV, 2500 mm

2XLPE cable

circuit. The longest cable sections are



approximately 1176 m, which is the record lengthfor this voltage in the UK to date.

A continuous monitoring system was applied to thecable system [6]. Partial discharges (PD) arecontinuously monitored at all joints andterminations, and at the same time the systemperforms measurement of oil pressure interminations and checks the condition of all sheath

voltage limiters (SVLs) located in the joint bays(Fig.6).

Figure 6 Schematic diagram of monitoring system

The concept of an applied continuous monitoringsystem is presented in Fig. 7. The signals fromdifferent sensors measuring partial discharges,distributed temperature, oil pressure interminations and sheath voltage limiters areacquired by multi-channel data acquisition units.

Figure 7 Concept of monitoring system

In case of PD signals, the acquisition unit performs

advanced pre-processing of the raw data. Thedisturbances are removed and main characteristicsof the PD signal are determined. The output of thedata pre-processing is transferred to a server thatenables long-term data storage. Advancedintelligent pre-processing reduces the amount ofdata for transmission over a communicationnetwork.

The separation of PD sources and effectivesuppression of external noise is achieved by theapplication of synchronous multi-channel (3PARD)evaluation techniques. The 3PARD diagram

visualizes the relation among amplitudes of asingle PD pulse in one phase and its crosstalkgenerated signals in the other two phases [7].

B. PD Acquisition System and Inductive PowerSupply

Inductive high frequency current transformers(HFCT) sensors are mounted on cross-bonding(CB) links and are used to detect PD directly at theaccessories. The PD monitoring system consists ofone four-channel, high-precision and modularacquisition unit for each accessory. The acquisitionunit is connected to a data concentrator. One data

concentrator collects monitoring data from two orthree acquisition units via fiber optic cables, and itroutes the data to a server. Pre-processingfunctions, such as band pass integration, gating,denoising and multi-source separation, are alreadyperformed in the data concentrator.

The active components of the monitoring systemrequire electrical power for several processes. Inaddition to the computer and communicationsdevices, pre-amplifiers or signal converters (forexample electrical to optical) located close to thesensors also have to be supplied with power.

The Inductive Power Supply provides thenecessary electronics to supervise and managethe dc current delivered at its output, depending onthe various HV cable current load situations (Fig.8). The PCBs and electronics elements are filteredand optimized to avoid any disturbances of the PDmeasurements close by.

Figure 8 Inductive Power Supply

C. Sheath Voltage Limiters Monitoring

Sheath Voltage Limiters minimize the transientvoltage across the screen separation of cross-

bonding joints during switching or lightningtransients and reduce the risk of damage. Theaccess to the SVLs is limited, so there is arequirement to continuously monitor their status,such as:

• Normal operation (below inception voltage);

• SVL is short circuited when the conductiveflashover trace is generated through the SVLvaristors;

• SVL is in open loop – totally damaged (activeelements destroyed-exploded).

An SVL is a non-linear resistor and, together withthe parallel cable screen at a cross-bonding linkcreates characteristic loop impedance. Thisimpedance will change according to the status ofthe SVL, and the operation of the SVL monitoringsystem is based on these changes. In themonitoring system, the PD acquisition units located



at the joint bay periodically “inject” signal pulsesusing their internal test generators. These signalspropagate through the HFCT sensors to the SVLimpedance loop and are collected by theacquisition unit. The Fourier transforms of injectedand returned signals are calculated, averaged andcompared. No special sensors are required for thescope, and a major part of SVL monitoring dataevaluation is performed within the monitoring

server. Differences can be easily analyzed anddistinguished by a spectral processing algorithmimplemented in the server software (Fig. 9).

Figure 9 Frequency signal response fordifferent status of SVL

D. Server and Software Architecture

The monitoring server receives data for analysis,display, and storage. The acquisition units areconfigured and remote-controlled by the monitoringsystem software. The software supports remoteaccess over TCP/IP. This allows operators toquickly react to detected problems and access thestored data from any remote location. The softwareis a highly modular, scalable distributed system. Its

system architecture consists of the Windows-based core part and the web-based control part.The core part of the monitoring software is realizedas windows services and runs continuously withoutany direct user interactions. The core systemimplements: Collection and persistence ofmeasurement; data post-processing and analysis;security tasks for data access and systemoperations; and external interfaces for dataexchange over Ethernet or field bus.

E. Trend Analysis

The monitoring system provides data from each ofthe acquisition units and oil pressure sensors inpermanent and periodic time intervals (Table I).

TABLE I DEFAULT TIME SCHEDULING FOR

DIFFERENT MEASURED VALUES

Mode

Value Permanent Periodic

Partial Discharge every 2-3 sec for 1 every1 h

Oil pressure atterminations

every 2- 3 sec once every1 h

SVL status -- once every8 h

During the permanent mode, the data is acquiredevery 2-3 seconds, compared with thresholdvalues and displayed in real-time in the graphicaluser interface. If this data is within normal margins,it will be colored in green. If the values exceedthresholds for "warning" or "alarm", they arecolored in yellow or red accordingly. Periodicmeasurements are initiated in equidistant timespans, such as every hour. The duration of the

periodic measurement is normally 1 minute. Duringthis time span all mentioned scalar values arecalculated and PRPD (phase resolved PD) and3PARD diagrams are acquired. This data is savedfor later post-processing and trend visualization.Unscheduled periodic measurements are triggeredin case of one or more measured quantitiesexceeding the threshold level.

PD activity is displayed as PRPD for eachphase/sensor and for each separated PD source,respectively. Trend diagrams of statisticalparameters such as PD magnitude, frequency of

occurrence of PD pulses, etc. are available.Suitable filter options enable the user to select thedata display according to his specific interest. Theuser can set limits that cause warning or alarmmessages to be generated when exceeded. Themeasured values are continuously compared withsignal levels (Fig.8). The measured quantities arecolor-coded based on their value related to pre-setwarning or alarm threshold levels. For example, ifthe detected PD level on any channel on one assetexceeds a configurable threshold, thecorresponding value will be drawn in red.

Figure 10 Graphic user interface

The SVL status is verified within the ServerSoftware by FFT-based spectral analysis of thesignal injected to the XB link loop impedance fromPD acquisition unit. A statistical model of “normal”SVL behavior is used for reference. The model isconstructed based on the SVL data collectedduring starting limited time period of cable systemoperation with different load conditions. SVL statusanalysis is based on statistical comparisonparameters between the model and current SVLmeasurements which clearly distinguish normaloperation of SVL from short circuited and openloop SVL.

F. Acceptance of the Monitoring System on site

The monitoring system was routinely tested in thefactory and later installed on-site (Fig. 10).



Figure 11 Installation of the system on site

The site check of monitoring system performancewas performed according to the following steps:

• Step 1: Verification of the functional readinessof the measuring system and of the monitoringserver

• Step 2: Verification of the synchronousbehaviour of the PD measuring system

• Step 3: Determination of PD impulseattenuation, damping and dispersion along thecable system.

The following parameters were determined: PDimpulse attenuation; damping and dispersion alongthe cable; velocity of the calibration signal in thecable; best frequency ranges for PDmeasurements at all PD units (with highest signalto noise ratio); and PD detection path divisionfactor for every chosen frequency range.

G. Maintenance of the system and customersupport

The monitoring system services and maintenancescheme is presented in Table II. The installedsystem elements, such as sensors, acquisitionunits, power supplies, batteries and fiber optic datatransmission network elements are periodicallyinspected and checked. These visual inspectionsand functional checks include adjustments, repairor minor maintenance activities. Such visualinspections are planned once every three years incoordination with the scheduled maintenanceactivity of the cable system. This requires accessto the system installations, including the cabletunnel, manhole, shafts and other related

substation facilities. It may also necessitate aneventual outage of the cable system, whichrequires respective scheduling efforts. Softwareupdates include periodic modifications, bug fixes,and enhancements with new features. If requestedby the system owner, regular checks andevaluation of data values as well as trending of theacquired partial discharges stored in the monitoringsystem database can be performed on a per cablesystem accessory basis. In the case of repeatedlyreported trending alerts or alarms, or specific PDevents detected by the monitoring system, outsidePD expert consultancy and support can be

requested by the system owner.

TABLE II MAINTENANCE SCHEME OF

MONITORING SYSTEM

Systemelemen

t

Maintenance scheme

Activi ty to beperformed

PeriodicityResponsibilit

y

Hardare

visual check yearly owner

functionality check every 3 yearsowner &systemprovider

Sofware updates every 3 years

system

provider

data evaluationperiodicreports

systemprovider

expert consultancyin case of PDevent

systemprovider

CONCLUSIONS

• The combination of resonance AC voltagetesting and distributed, synchronous PDmeasurements at all cable accessorieshas proven highly effecive for afterinstallation testing of HV XLPE cable

systems.• A continuous PD monitoring system

provides actionable data to supportmaintenance on a condition-based ratherthan time-based plan to extend the life ofthe HV asset

• Separation of PD sources and suppressionof external noise is effectively performedby the multi-channel evaluation techniquesof the monitoring system

• The Inductive Power Supply provides thenecessary power to the monitoringequipment to supervise the 400 kV cable

system. The dc current delivered isdepending on the HV cable load situations,but even at very low load the power issufficient to run the monitoring system;

• To verify the status of SVL, the system canutilize the CB link loop impedance;

• A modular, distributed monitoring softwaresystem allows reliable long term storage ofmonitoring data and provides access viaweb interface.

• The monitoring system provider supportsthe asset owner in all stages of themonitoring project, from system design,

installation and periodic mainenance, totraning and data evaluation support.

References[1] M. Krüger et al."Partial Discharge

Measurement and Monitoring on High VoltageXLPE Cables", CIGRE Symposium Auckland,2013

[2] IEC 60840, Edition 3.0 2004-04 “Power cableswith extruded insulation and their accessoriesfor rated voltages above 30 kV (Um= 36 kV) upto 150 kV (Um= 170 kV) – Test methods andrequirements”

[3] IEC 62067, Edition 1.1 2006-03 “Power cableswith extruded insulation and their accessoriesfor rated voltages above 150 kV (Um= 170 kV)up to 500 kV (Um= 550 kV) – Test methodsand requirements”



[4] IEC 60270 "Partial discharge measurements",3

rd Edition, 2000.

[5] IEC 60885-3, “Electrical test methods forelectric cables Part 3: Test methods for partialdischarge measurements on lengths ofextruded power cable”

[6] D. Gieselbrecht, W. Koltunowicz, A. Obralic,et. al.: “Monitoring of 420 kV XLPE CableSystem in Underground Tunnel”, IEEE

International Conference on ConditionMonitoring and Diagnosis 23-27 September2012, Bali, Indonesia

[7] W. Koltunowicz and R. Plath, 2008,"Synchronous multi-channel PDmeasurements", IEEE Transactions onDielectrics and Electrical Insulation, vol.15,no.6, pp. 1715-1723, 2008.

The authors

Dr.hab. Wojciech Koltunowicz received the M.S.,PhD and Dr. hab. degrees in electrical engineering

from the Warsaw University of Technology in 1980,1985 and 2004, respectively. From 1984 to 1987,he was with Institute of Power in Poland, as aresearch scientist in the High Voltage Department.From 1987 to 2007 he was with CESI, Italy, wherehe was mainly involved in HV testing anddiagnostics of HV equipment. In 2007, he joinedOMICRON, where he is involved in monitoring ofHV equipment. He is Secretary of CIGRE AdvisoryGroup D1.03 “Insulating Gases” , WG D1.25 andMember of AG D1.02 “High Voltage and HighCurrent Test and Measuring Technique andDiagnostic” and WGs D1.28, D1.37 and D1.51. He

is also member of IEC TC42 WG14. He is authorof dozens of international reports.

Dr.Michael Krüger is head of engineering serviceswith OMICRON electronics GmbH, Austria. Hestudied electrical engineering at the University of Aachen (RWTH) and the University ofKaiserslautern (Germany) and graduated in 1976(Dipl.-Ing.). In 1990 he received the Dr. techn. fromthe University of Vienna. Michael Krüger has morethan 25 years of experience in high voltageengineering and insulation diagnosis. He ismember of VDE and IEEE.



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PD Testing and Monitoring of HV XLPE Cable Systems ENU